The opportunity was lost when the Jordanian authorities arrested a key-negotiator despite earlier agreeing to allow al Qaeda leaders to contact ISIS.

Even till the last hours, prominent jihadi clerics including Guantanamo detainees tried hard to save Peter Kassig, but failed after the Jordanian intelligence arrested a key negotiator, who could have helped in rescuing the US citizen from ISIS.

According to a report in The Guardian, US counter-terrorism officials along with FBI had several rounds of negotiations involving two of the world's most prominent jihadi clerics as well as former Guantánamo detainees in mid-October.

Controversial New York lawyer Stanley Cohen, who represented Osama bin Laden's son-in-law and members of Hamas in US courts, was the one who established contact with the al-Qaeda "spiritual" guides, who also have good influence among the ISIS cadres.

Cohen contacted two senior clerics - Abu Muhammed al-Maqdisi and Abu Qatada - aligned with al-Qaeda, to negotiate with ISIS. The "spiritual leaders" joined the negotiations stating that they not only want to save Kassig but also would try to stop ISIS from abducting and executing civilians.

Maqdisi, Abu Qatada and other senior religious figures promised that in return they would stop publicly denouncing ISIS as "deviants."

However, during the negotiations, Jordanian security services arrested Maqdisi for "using the Internet to promote and incite views of jihadi terrorist organisations."

The arrest not only shocked Cohen, but also led to the collapse of the entire negotiation process, as in the beginning, both the US and Jordan had agreed "not to bring up charges on the Al Qaeda members for establishing contact with ISIS."

Cohen said, "I feel we lost a golden opportunity to not only save Kassig but other potential hostages."

Kassig was abducted in October 2013 by the Islamic State (ISIS) militants in Syria, where he had gone to carry out humanitarian work. During his captivity, he converted to Islam and assumed the name of Abdul-Rahman Kassig.

He was beheaded on 16 November. His execution was recorded and the video "Although The Unbelievers Dislike" was uploaded on social media the same day.